Alterations in Protein Synthesis Following Transplantation of Mouse 8-Cell Stage Nuclei to Enucleated 1-Cell Embryos

Enucleated mouse 1-cell embryos arrest development at the 2-cell stage following transplantation of cleavage stage nuclei. Earlier studies employing one-dimensional protein gel electrophoresis failed to reveal obvious differences in gene expression in the manipulated embryos that might account for this block. We report here the results of a quantitative, two-dimensional gel electrophoretic analysis that reveals at least 50 alterations in protein synthesis in the 8 → 1-cell nuclear transplant embryos. Approximately half of these alterations involve proteins that normally decrease in synthesis between the 2-cell and 8-cell stages and half involve proteins that are synthesized constitutively between these two stages. These results are the first to reveal significant biochemical alterations that accompany the morphological and cytological differences previously described and indicate that the 8-cell stage nucleus is unable to completely recapitulate the normal progression of changes in protein synthesis pattern that occur during the 2-cell stage. Our results indicate that developmental arrest may result from a combination of (1) failure to re-activate genes that normally become repressed between the 2-cell and 8-cell stages and (2) failure to up-regulate genes that become repressed during reprogramming of the 8-cell nucleus by the 1-cell cytoplasm. The failure to up-regulate the synthesis of proteins expressed at both the 2-cell and the 8-cell stages may indicate that, for some genes, a transcriptionally inactive state, possibly related to a particular chromatin configuration, may serve a protective function by restricting access of factors that can permanently reduce the ability of a gene to be expressed. This may partially account for the need to delay the widespread activation of zygotic gene transcription until the 2-cell stage, when much of the nuclear remodeling that occurs postfertilization is complete.